Data received over a noisy channel, such as a radio channel between a base station and a mobile device, e.g., according to 3GPP Long Term Evolution (LTE) or LTE Advanced, is partly protected by a Hybrid Automatic Repeat Request (HARQ) protocol. The HARQ protocol combines forward error correction provided by a channel code with error detection provided by a Cyclic Redundancy Check (CRC). A mobile device receiving a transport block decodes the channel code. Depending on channel quality, not all transmission errors can be corrected by the forward error correction, which is detected by the mobile device as a CRC failure. Softbits representing such an incorrectly received transport block are stored in a HARQ buffer and combined with softbits of a later retransmission related to the same HARQ process.
For using the channel efficient, a plurality of HARQ processes is executed in parallel on a Media Access Control (MAC) sublayer of the mobile device in a stop-and-wait mode. While one or more HARQ processes are waiting for the retransmission of an incorrectly received transport block, another HARQ process receives data on the channel. Therefore, the HARQ buffer at the mobile device has to store a large amount of softbits. For fast data access, the HARQ buffer is conventionally implemented in On Chip Memory (OCM). However, the OCM is expensive and consumes chip space.
In order to reduce the usage of OCM, existing implementations overbook the memory HARQ buffer. The document WO 2010/069379 describes such a dynamic HARQ buffer management. The HARQ buffer manager administrates the OCM by assigning memory requests to physical resources at the costs of sacrificing valid HARQ data, if no free physical memory space is available in the OCM.
However, the loss of HARQ data can significantly reduce data rates in at least certain situations, e.g., on higher layers of the protocol stack. For example, the data throughput on a Transport Control Protocol (TCP) link is low under critical channel condition, when an initial Block Error Rate (BLER) is at 0.2 or higher. The conventional dynamic HARQ buffer management overwrites valid HARQ data, i.e., softbits of a transport block, the decoding of which initially failed. This leads to retransmissions of the failed transport block that are also not successfully decoded with high probability due to the lacking HARQ data. After a certain number of failed retransmissions, the HARQ protocol on the MAC sublayer controlling the physical layer stops the retransmissions and the TCP enforces the reduction of the data rate.